The invention relates to a process for the preparation directly from polymerization of fluoropolymers aqueous dispersions, having a multimodal or bimodal distribution of the particle sizes and to the dispersion obtainable by a high conversion preparation process; said dispersions having an average diameter in the range 100 nm-400 nm, preferably 150 nm xe2x88x92300 nm, the fraction of small particles having diameter between 10 and 100 nm is in the range 1.5-35% by weight, preferably 3-259 by weight; the ratio between the average diameter of the small particles with respect to the average diameter of the dispersion obtained from the polymerization being in the range 0.025-0.6.
In particular the invention relates to a radical polymerization process of tetrafluoroethylene (TFE) and copolymers thereof in aqueous emulsion, using a specific dosage of surfactants of which at least one is a component of a microemulsion.
From the literature (J. G. Brodnyan, Trans. Soc. Rheology, 12:3 (1968), 357; J. S. Chong, E. B. Christiansen, A. D. Baer, J. Appl. Pol. Sci., v.15 (1971), 2007; P. -L. Kuo, C. -J. Chen, J. Polymer Sci. Pol. Chemistry, 31 (1993), 99; C. Chang, R. L. Powell, J. Rheol., 38 (1994), 85; Emulsion polymerization and emulsion polymers. Ed. A. Lovell, M. S. El-Aasser, J. Wiley and Sons, Chichester, 1997, p. 450) it is known that polymeric latexes with a bimodal distribution of the particle diameters with respect to those with a monomodal distribution have a lower viscosity, the concentration being equal. This allows to obtain latexes having a higher concentration not exceeding a certain viscosity, for example up to 74% by volume with a viscosity lower than 1,000 cP. This leads, for example, to a better capability of the film formation and better tensile properties of the film.
Lately (F. Chu, J. Guillot, A. Guyot, Colloid. Polym. Sci., 276 (1998), 305; F. Chu, J. Guillot, A. Guyot, J. Applied Polymer Sci., 70 (1998), 2667) it has been shown for the polymers based on styrene, butyl acrylate and methylmethacrylic acid, that latexes with multimodal, in particular trimodal, distributions of the particle diameters, have more advantageous rheological and colloidal properties with respect to bimodal latexes and much more advantageous with respect to monomodal latexes. For example, a latex can be obtained having a solid content up to 80% by volume and minimum viscosity in the case of trimodal distribution of the particle diameters with ratio by weight among large/average/small particles equal to 80/10/10.
The above mentioned literature and U.S. Pat. No. 4,657,966, U.S. Pat. No. 4,385,157, U.S. Pat. No. 5,668,207 refer to hydrogenated polymer latexes and fluoropolymer latexes are not mentioned.
In U.S. Pat. No. 5,576,381 it is described how to obtain PTFE latexes with a bimodal distribution of the particle diameters by mixing a latex having a particle diameter in the range 180 nmxe2x80x94400 nm with another latex having an average diameter in a ratio from 0.3 to 0.7 with respect to the particle diameter of the first latex. The process requires the carrying out of two separate polymerization operations with a subsequent mixing in a specific ratio. This complicates the production process, since it requires the separate production of two types of latex and the increase of the storage volumes. Besides, the latex polymerization process with small particles requires to work at a low polymerization conversion, i.e. with low productivity, and it does not allow to obtain the particles having a diameter lower than 100 nm with the typical conversions of the industrial scale. As operating example the preparation of a latex having an average particle diameter of 100 nm the conversion being 10% by weight, is reported. In this patent it is stated that the fluoropolymer latexes with bimodal distribution of the particle sizes can be obtained by carrying out a polymerization reaction with subsequent additions of a surfactant. No example is reported on this aspect. From the examples, bimodal or multimodal distributions of the particle diameters are not obtained from the polymerization. On the other hand, it is known that subsequent additions of surfactants commonly used for the polymerization of tetrafluoroethylene based fluoropolymers, as for example described in U.S. Pat. No. 3,009,892 and U.S. Pat. No. 3,391,099 with the purpose to increase the latex stability and the conversion, do not give bimodal or multimodal distributions of the particle diameters. Experiments carried out by the Applicant (see comparative Examples) have shown that by using during the PTFE polymerization subsequent additions of ammonium perfluorooctanoate or of ammonium salt of perfluoropolyether acid as surfactants, multimodal distributions are not obtained.
The patent application WO 98/58984 mentions the possibility to obtain fluoropolymer latexes having a bimodal distribution of the particles directly from the polymerization but it is not indicated anyway how this result can be obtained.
U.S. Pat. No. 4,864,006 uses a microemulsion based on (per)fluoropolyethers for the polymerization of fluorinated polymers and indicates that all the surfactant necessary for the polymerization is fed into the reactor under the form of microemulsion. Besides, it is indicated that the whole microemulsion must be introduced into the reactor before the starting of the reaction. The amount of the used microemulsion must be higher than 0.05 ml of perfluoropolyether for 1 liter of water. Tests carried out by the Applicant (see comparative Examples) have shown that by operating under the conditions indicated in the patent dispersions with multimodal distributions of the particle diameters are not obtained.
In U.S. Pat. No. 4,380,618, U.S. Pat. No. 5,789,508 and WO 97/08214 for the PTFE polymerization and copolymers thereof surfactants of formula:
Fxe2x80x94(CF2xe2x80x94CF2)nxe2x80x94CH2xe2x80x94CH2xe2x80x94SO3M, wherein M=H, NH4, Na, Li, K are used. Multimodal distributions of the particle diameters are not obtained (see the Examples).
In U.S. Pat. No. 4,038,230 it is described how to obtain large spherical PTFE particles by using a non reactive perfluorinated oil phase, for example perfluorobutane, together with the surfactant. Multimodal distributions of the particle diameters are not reported.
In U.S. Pat. No. 3,526,614 in order to obtain a sufficiently large particle diameter, the addition of polivalent metal salts, such as Zn2+, is used. This technique allows to control the final diameter of the PTFE particles, but multimodal distributions of the particle diameters are not reported.
U.S. Pat. No. 5,523,346 uses for the PTFE polymerization a microemulsion based on polymerizable unsaturated liquid monomers in order to obtain small PTFE particles. This method does not allow to obtain multimodal distributions of the particle diameters. Besides, it is well known that by operating under these conditions it is extremely dangerous due to the explosiveness of the liquid tetrafluoroethylene.
In patent application WO 96/22315 a perfluorinated microemulsion is used in order to obtain small particles of thermoprocessable fluoropolymers. Multimodal distributions of the particle diameters are not obtained.
U.S. Pat. No. 3,925,292 and U.S. Pat. No. 5,789,083 relate to a mixture formed by a dispersion of PTFE and by a dispersion of a thermoplastic copolymer of tetrafluoroethylene. In said patents it is neither reported nor described that the obtained single latexes have a multimodal distribution of the particle diameters. It is not indicated that said mixture can be directly obtained from the polymerization process.
In EP 969,027 in the name of the Applicant, a polymerization process in microemulsion to obtain PTFE particles or PTFE copolymers having a size 5-60 nm, is described. Bimodal distribution is not described.
The need was felt of a polymerization technique able to give PTFE dispersions having a multimodal distribution as defined below.
It is therefore an object of the present invention a process for the preparation of a dispersion based on tetrafluoroethytlene (TFE) homopolymers, or based on copolymers thereof with one or more monomers containing at least one unsaturation of ethylene type in amounts from 0 up to 10% by moles, preferably from 0 to 2% by moles, said dispersion having a multimodal or bimodal, preferably bimodal, distribution of the particle diameters, the dispersion having an average diameter in the range 100 nm-400 nm, preferably 150 nm-300 nm, the fraction of the small particles having diameter between 10 and 100 nm being in the range 1.5-35% by weight calculated on the dry product, preferably 3-25% by weight; the ratio between the average diameter of the small particles with respect to the average diameter of the dispersion obtained from the polymerization being in the range 0.025-0.6; said process comprising:
(a) preparation of an aqueous microemulsion comprising fluorinated surfactants and (per)fluoropolyether (PFPE) oils having non reactive, preferably (per)fluorinated, end groups, optionally containing 1 or more H and/or Cl atoms instead of fluorine;
(b) feeding of the microemulsion into the polymerization reactor before the beginning of the polymerization or when polymerization has started;
(c) feeding of at least one fluorinated, preferably perfluorinated, surfactant into the reactor before the starting of the polymerization or when polymerization has started;
(d) addition of water (reaction medium) into the reactor, reactor pressurization with gaseous TFE, optional addition of comonomers, chain transfer agents, stabilizers;
(e) addition of the initiator, and optionally during the polymerization of further amounts of comonomers, initiators, chain transfer agents;
(f) discharge from the reactor of the polymer latex; before the starting of the reaction at least a part of the (b) and/or (c) feedings must be carried out.
With the process of the invention a Latex containing a high concentration of polymer, generally higher than 259 by weight, preferably higher than 30W by weight, is obtained.
The Applicant has surprisingly found that by using a dosage system of at least two surfactants of which one is a component of a microemulsion (a) based on (per)fluoropolyethers and the other is a fluorinated surfactant not in microemulsion, PTFE latexes having a multimodal or bimodal distribution of the particle diameters are directly obtained from the polymerization process of the invention.
The fraction and the diameter of the small particles are obtainable from the supernatant layer of the polymerization latex subjected to ultracentrifugation.
Preferably the amount of the surfactant fed before the starting of the polymerization must exceed 0.05 grams for liter of water, while the total amount of the fed surfactant is generally in the range 0.3-10 g for liter of water.
The amount of the fluorinated surfactant in the microemulsion (a) is in the range 1-80% by weight, preferably 1-45% by weight, with respect to the total weight of surfactants (a)+(c).
The fluorinated surfactant used in step (c) is preferably anionic.
It is possible to check that the granulometric distribution curve of the latex particles has at least two separate peaks (bimodal distribution) by a scanning electronic microscope (SEM) or transmissiom electronic microscope (TEM) or by an atomic force microscope (AFM). Another method is that to use one or more ultracentrifugations of the polymerization latex.
The latex discharged from the reactor can be subjected, if desired, to the usual post-treatments in connection with the specific uses. For example, the discharged dispersion can be coagulated to obtain fine powders having the multimodal distribution according to the present invention. The latex concentration, carried out for example by clouding or ultrafiltration, can also be mentioned.
In order to obtain trimodal or higher distributions of the particles, a fluorinated surfactant (c) and the microemulsion (a) must be fed into the reactor at different reaction progress degrees. For example a feeding of the microemulsion (a) and of the surfactant (c) before the starting of the reaction, a further feeding of microemulsion a) when more than 50% of tetrafluoroethylene has reacted.
The Applicant has found that the various obtained populations of the polymer have different not only the particle diameters, but also some properties, such as molecular weight, crystallinity, etc. can be different.
For example, by using together with the microemulsion a specific dosage of comonomers, initiators and chain transfer agents, latexes can be obtained which besides containing a fraction of large particles formed by PTFE or modified PTFE, having a sufficiently high molecular weight such as to be not thermoplastic, contain at least one fraction of small particles formed by a thermoplastic fluoropolymer based on TFE, i.e. a fluoropolymer having a viscosity of the melt lower than 107 poise, preferably from 103 to 107 poise. In order to obtain the large particle fraction having a sufficiently high molecular weight, the microemulsion dosage with initiators, comonomers and chain transfer agents must be fed at a reaction progress degree of at least 60%, preferably 80%.
The results of the invention are particularly surprising since the particle nucleation mechanism in polymerization processes in emulsion or in microemulsion is very complex and it is not clarified. The processes leading to the formation of small and large particles influence each other. It is surprising that by the process of the invention multimodal distributions as above defined are obtained.
The dispersions obtained by the process of the invention, as such or after post-treatment, can be used for the coating of surfaces of organic and/or inorganic polymers, of metals or ceramics, etc. with the advantage to have a better filming with respect to the dispersions having a monomodal distribution of the particles. Besides, the films obtained from the invention dispersions, show also higher tensile properties with respect to the commercial monomodal dispersions having a diameter in the range 180-400 nm.
The microemulsions (a) used in the process of the present invention are described in U.S. Pat. No. 4,864,006 and U.S. Pat. No. 4,990,283, herein incorporated by reference, wherein instead of the indicated non reactive end groups, also hydrofluoropolyethers having one or both end groups containing one H atom, or having one or more chlorine atoms instead of fluorine in the chain end groups, can be used.
The perfluoropolyether oil phase of the microemulsion a) is present in an amount from 0.002 to 10 ml for liter of water, preferably from 0.01 to 2.5 ml.
The number average molecular weight of the perfluoropolyethers (PFPE) which can be used for the preparation of the microemulsion (a) is in the range 350-8,000, preferably 500-1,000. The perfluoropolyether chain comprises repeating units having at least one of the following structures, statistically placed along the chain:
(CFXO) (CF2CF2O), (CF2CF2CF2O), (CF2CF2CF2CF2C), (CR4R5CF2CF2O), (CF (CF3) CF2O), (CF2CF (CF3)O),
wherein
X=F, CF3;
R4 and R5, equal to or different from each other, are selected from H, Cl, or perfluoroalkyl from 1 to 4 carbon atoms.
In particular PFPE can have one of the following structures:
(1) TO-(CF2O)axe2x80x2xe2x80x94(CF2CF2O)bxe2x80x2xe2x80x94T
with axe2x80x2/bxe2x80x2 in the range 0.5-2, extremes included, axe2x80x2 and bxe2x80x2 being integers such as to give the above indicated molecular weight;
(2) TO-(C3F6O)rxe2x80x94(C2F4O)bxe2x80x94(CFXO)t-T
with r/b=0.5xe2x88x922.0; (r+b)/t is in the range 10-30, b, r and t being integers such as to give the above indicate molecular weight, X has the above indicated meaning;
(3) TO-(C3F6O)rxe2x80x2xe2x80x94(CFXO)txe2x80x2-T
txe2x80x2 can be 0;
when txe2x80x2 is different from 0 then rxe2x80x2/txe2x80x2=10xe2x88x92100, preferably 10-30; rxe2x80x2 and txe2x80x2 being integers such as to give the above indicated molecular weight; X has the above indicated meaning;
In the above indicated formulas:
(C3F6O)xe2x80x94 can represent units of formula 
xe2x80x94(CF(CF3)CF2O)xe2x80x94 and/or xe2x80x94(CF2xe2x80x94CF(CF3)O)xe2x80x94
The T end groups, equal to or different from each other, are (per)fluoroalkyl groups selected from: xe2x80x94CF3, xe2x80x94C2F5, xe2x80x94C3F7, xe2x80x94CF2Cl, xe2x80x94C2F4Cl, xe2x80x94C3F6Cl; optionally one or two F atoms, preferably one, can be replaced by H.
The indicated fluoropolyethers are obtainable by the well known processes in the prior art, see for example the following patents herein incorporated by reference: U.S. Pat. Nos. 3,665,041, 2,242,218, 3,715,378, and EP 239,123.
PFPEs are commercially known, for example FOMBLIN(copyright) commercialized by Ausimont. The preferred ones among them are:
CF3O(C3F6O)n(CF2O)mCF3 
wherein n and m are such as to give the above indicated molecular weight values; m/n is in the range 0.01-0.5.
The fluorinated surfactants used in the present invention can be selected from the following:
TO(C3F6O)n(CF2O)mCF2COOM 
wherein M=H, NH4, Na, Li, K and n can range from 1 to 6, T as above defined; m/n is in the range 0.05-0.1
CF3(CF2)nCOOM
wherein n can range from 4 to 12
Fxe2x80x94(CF2xe2x80x94CF2)nxe2x80x94CH2xe2x80x94CH2xe2x80x94SO3M
wherein M=H, NH4, Na, Li, K and n can range from 2 to 5.
Preferably the fluorinated surfactants of the microemulsion (a) have structure (IA).
The functionalized fluoropolyethers of structure (IA) are obtained for example according to the processes mentioned in EP 148482, U.S. Pat. No. 3,810,874. The surfactants of structure (IIA) and (IIIA) are commercial products, for example FLUORAD(copyright) FC1013, FORAFAC(copyright) 1033D.
Comonomers having an ethylene unsaturation which can be used are of both hydrogenated and fluorinated type; among the hydrogenated ones, ethylene, propylene, acrylic monomers, for example methylmethacrylate, (meth) acrylic acid, hydroxyethylenacrylate, styrene monomers, such as for example styrene, can be mentioned. Among the fluorinated comonomers it can be mentioned:
C3-C8 perfluoroolefins, such as hexafluoropropene (HFP);
C2-C8 hydrogenated fluoroolefins, such as vinyl fluoride (VF), vinylidene fluoride (VDF), trifluoroethylene, perfluoroalkylethylene CH2xe2x95x90CHxe2x80x94Rf, wherein Rf is a C1-C6 perfluoroalkyl, such as hexafluoroisobutene;
C2-C6 chloro- and/or bromo- and/or iodo-fluoroolefins, such as chlorotrifluoroethylene (CTFE);
(per)fluoroalkylvinylethers (PAVE) CF2xe2x95x90CFORf, wherein Rf is a C1-C6 (per)fluoroalkyl, for example CF3, C2F5, C3F7;
(per)fluoro-oxyalkylvinylethers CF2xe2x95x90CFOX, wherein X is a C1-C12 alkyl, or a C1-C12 oxyalkyl, or a C1-C12 (per)fluorooxyalkyl having one or more ether groups, for example perfluoro-2-propoxy-propyl;
fluorodioxoles, preferably perfluorodioxoles.
During the polymerization the typical temperatures and pressures of the TFE polymerization processes are used.
The present invention will be better illustrated by the following Examples, which have a merely indicative but not limitative purpose of the scope of the invention itself.